1. Field of the Invention
The present invention relates generally to methods and apparatus for the detection of the degree of airway obstruction of a human patient, which can be useful in the diagnosis and treatment of sleep disorder breathing and other respiratory and pulmonary difficulties.
2. Background Information
Disorders of excessive sleepiness present particular health-care concerns. Patients suffering from these disorders experience drowsiness and the need or desire to take naps during the day, and such patients present a history of divorces, employment problems, and automotive accidents.
Among the most common types of sleep disorder breathing is sleep apnea, in which patients experience a partial or complete interruption of air flowing into the lungs for periods exceeding ten seconds. Between 1% and 15% of the population is believed to suffer from this condition. Sleep apnea can cause repeated disruption or even cessation of rapid eye movement (REM) sleep, which can cause irritability and a reduction in the ability to memorize information.
There are three recognized types of sleep apnea. Central sleep apnea is characterized by the suspension of all respiratory movement and is generally believed to be neurological in origin. Obstructive sleep apnea is characterized by the collapse of the upper airways during sleep. The third type of sleep apnea is a combination of central and obstructive sleep apnea and is known as mixed apnea.
Obstructive sleep apnea appears to be the most common form of sleep apnea and occurs when the upper respiratory airway of the patient collapses because the tonal activity of the pharyngeal smooth muscle fails to maintain the patency of the airway. Although sporadic and brief airway collapses or obstructive events are not uncommon in the normal adult population, it is considered pathological when obstructive apnea episodes last more than ten seconds and occur over seven-to-ten times per hour.
A symptom indicative of the onset of obstructive sleep apnea is pharyngeal wall vibration, commonly known as snoring when audible. Early detection of pharyngeal wall vibration, and prophylactic treatment of the condition, can lead to successful treatment of obstructive sleep apnea. Surgical treatments of obstructive sleep apnea are successful when anatomical abnormalities appear to be the principal cause of obstructive sleep apnea. Non-surgical treatments are successful as well.
One successful non-surgical treatment method is the use of continuous positive airway pressure (CPAP) apparatus. CPAP apparatus administers air or respiratory gas to the patient's airways at a slightly positive pressure level (5 to 20 cmH.sub.2 O), which maintains the patency of the respiratory airways. The pressure exerted by CPAP apparatus is believed to act as a pneumatic splint for the upper airway. The low-level pressure of the CPAP apparatus is selected after study of the patient in a sleep laboratory. The selected pressure from the sleep study is referred to as "prescribed" CPAP pressure. Although CPAP apparatus is an effective treatment in 75% of treated patients, the positive airway pressure delivered throughout sleep can cause patient discomfort, including airway pain and dehydration.
All treatments of sleep disorder breathing depend, to one degree or another, upon accurate diagnosis of the degree and type of airway obstruction. The foregoing methods and apparatus detect airway obstruction or other precursors to apneic events, but in a relative sense only (relative to previous inhalations and exhalations). While they are useful therapeutic tools, their ability to detect and diagnose the degree of airway obstruction in an abstract or absolute sense, as a diagnostic tool, is limited. Indeed, most provide no easy way of communicating this information directly to the treating physician or therapist.
Attempts have been made in the past to estimate the degree of obstruction of the airway of a patient's respiratory system. One technique is known as the forced oscillation technique (FOT). In this technique, an audio loudspeaker is coupled to the airway via a rigid mounting. A probing signal is generated by the loudspeaker and propagated into the airway. Respiratory impedance then is calculated from analysis of pressure and flow signals measured at the airway opening. Due to the sensitivity of the instruments used, the patient is typically awake, in an upright sitting position, and may not breathe during measurement.
Another technique for estimating airway obstruction degree or impedance is known as the airway resistance technique (ART). In this technique, a pressure pulse generated by a loudspeaker is passed to the airway through a straight, rigid tube. By analyzing the acoustic characteristics of the echo (reflected and refracted sound wave), the changes in the effective cross-section or diameter of the airway can be measured.
Both of these techniques are minimally invasive in that they do not require insertion of nasal, pulmonary, or interpleural catheters and the like. However, both methods restrict patient movement through the use of rigid mountings and must be undertaken under fairly controlled circumstances, such as seated and without the patient breathing (in the case of FOT) . Many types of breathing disorders, e.g. sleep apnea, occur during sleep or other conditions perhaps not easily reproduced in the laboratory.
A need exists, therefore, for methods and apparatus for the detection and diagnosis of the degree and character of the airway obstruction of a patient that are minimally invasive and permit diagnosis during normal patient activity, preferably sleep.